1. Field of the Invention
The present invention relates to protective granules for asphalt roofing shingles, coating compositions for preparing roofing granules, roofing granules, roofing shingles employing such granules, and processes for makings such granules and shingles.
2. Brief Description of the Prior Art
Pigment-coated mineral rocks are commonly used as color granules in roofing applications to provide aesthetic as well as protective functions to the asphalt shingles. Roofing granules are generally used in asphalt shingles or in roofing membranes to protect asphalt from harmful ultraviolet radiation.
Roofing granules typically comprise crushed and screened mineral materials, which are subsequently coated with a binder containing one or more coloring pigments, such as suitable metal oxides. The binder can be a soluble alkaline silicate that is subsequently insolubilized by heat or by chemical reaction, such as by reaction between an acidic material and the alkaline silicate, resulting in an insoluble colored coating on the mineral particles. For example, U.S. Pat. No. 1,898,345 to Deming discloses coating a granular material with a coating composition including a sodium silicate, a coloring pigment, and a colloidal clay, and heating below the fusing temperature of sodium silicate, and subsequently treating with a solution, such as a solution of calcium or magnesium chloride, or aluminum sulphate, that will react with the sodium silicate to form an insoluble compound. Similarly, U.S. Pat. No. 2,378,927 to Jewett discloses a coating composition for roofing granules consisting of sodium silicate, and clay or another aluminum-bearing compound such as sodium aluminate, or cryolite or other insoluble fluorides such as sodium silicofluoride, and a color pigment. The coating is then heat cured at a temperature above the dehydration temperature of the coating materials but below the fusion temperature at which the combination of materials fuses, thus producing a non-porous, insoluble weather-resistant cement. Preparation of colored, coated roofing granules is also disclosed for example, in U.S. Pat. No. 2,981,636 of Lodge et al. The granules are employed to provide a protective layer on asphaltic roofing materials such as shingles, and to add aesthetic values to a roof.
Pigments for roofing granules have usually been selected to provide shingles having an attractive appearance, with little thought to the thermal stresses encountered on shingled roofs. However, depending on location and climate, shingled roofs can experience very challenging environmental conditions, which tend to reduce the effective service life of such roofs. One significant environmental stress is the elevated temperature experienced by roofing shingles under sunny, summer conditions, especially roofing shingles coated with dark colored roofing granules. Although such roofs can be coated with solar reflective paint or coating material, such as a composition containing a significant amount of titanium dioxide pigment, in order to reduce such thermal stresses, this utilitarian approach will often prove to be aesthetically undesirable, especially for residential roofs.
Mineral surfaced asphalt shingles, such as those described in ASTM D225 or D3462, are generally used in steep-sloped roofs to provide water-shedding function while adding aesthetically pleasing appearance to the roofs. The asphalt shingles are generally constructed from asphalt-saturated roofing felts and surfaced by pigmented color granules, such as those described in U.S. Pat. No. 4,717,614. Asphalt shingles coated with conventional roofing granules are known to have low solar heat reflectance, and hence will absorb solar heat especially through the near infrared range (700 nm-2500 nm) of the solar spectrum. This phenomenon is increased as the granules covering the surface become dark in color. For example, while white-colored asphalt shingles can have solar reflectance in the range of 25-35%, dark-colored asphalt shingles can only have solar reflectance of 5-15%. Furthermore, except in the white or very light colors, there is typically only a very small amount of pigment in the conventional granule's color coating that reflects solar radiation well. As a result, it is common to measure temperatures as high as 77° C. on the surface of black roofing shingles on a sunny day with 21° C. ambient temperature. Absorption of solar heat may result in elevated temperatures at the shingle's surroundings, which can contribute to the so-called heat-island effects and increase the cooling load to its surroundings. It is therefore advantageous to have roofing shingles that have high solar reflectivity in order to reduce the solar heat absorption.
The surface reflectivity of an asphalt shingle largely depends on the solar reflectance of the granules that are used to cover the bitumen. Roofing granules are typically produced by pigmenting inert mineral particles with metal-silicate binders and clays as a latent heat reactant at an elevated temperature, for example, such as those described in U.S. Pat. No. 2,981,636.
In order to reduce the solar heat absorption, one may use light colored roofing granules which are inherently more reflective towards the solar radiation. White pigment containing latex coatings have been proposed and evaluated by various manufacturers. However, consumers and homeowners often prefer darker or earth tone colors for their roof. In recent years, there have been commercially available roofing granules that feature a reflective base coat (i.e., a white coat) and a partially coated top color coat allowing the reflective base coat to be partially revealed to increase solar reflectance. Unfortunately, these granules have a “washed-out” color appearance due to the partially revealed white base coat.
Other manufactures have also proposed the use of exterior-grade coatings that were colored by infrared-reflective pigments for deep-tone colors and sprayed onto the roof in the field. U.S. Patent Application Publication No. 2003/0068469 A1 discloses an asphalt-based roofing material comprising mat saturated with an asphalt coating and a top coating having a top surface layer that has a solar reflectance of at least 70%. U.S. Patent Application Publication No. 2003/0152747 A1 discloses the use of granules with solar reflectance greater than 55% and hardness greater than 4 on the Moh's scale to enhance the solar reflectivity of asphalt based roofing products. However, there is no control of color blends and the novel granules are typically available only in white or buff colors. U.S. Patent Application Publication No. 2005/0074580 A1 discloses a non-white construction surface comprising a first reflective coating and a second reflective coating with total direct solar reflectance of at least 20%.
Also, there have been attempts in using special near-infrared reflective pigments in earth-tone colors to color roofing granules for increased solar reflectance. However, the addition of kaolin clays, which are used to make the metal-silicate binder durable through heat curing, inevitably reduce the color strength or the color intensity of the pigment. As a result, dark-colored or black granules (L* less than 30) that have good solar reflectance (greater than 20 percent) cannot be produced using conventional roofing granule manufacturing processes with clay as a latent reactant.
Colored roofing granules can also be prepared using a metal silicate binder without adding clay and curing the binder at temperatures greater than glass sintering temperature, or through a “pickling” process by applying acid. However, these alternatives require either very high temperatures, or the use of corrosive chemicals, and in many cases could result in loss of color due to pigment degradation by the acid. In the alternative, a non-silicate binder, such as a synthetic polymeric binder, can be used to coat the inert mineral materials in order to produce roofing granules with dark colors and high solar reflectance. However, the long-term durability and cost for polymeric coatings are not as advantageous as the silicate binders.
Therefore, it would be advantageous to have a method of producing dark-colored roofing granules with high solar reflectance using a metal-silicate binder. There is a continuing need for roofing materials, and especially asphalt shingles, that have improved resistance to thermal stresses while providing an attractive appearance. In particular, there is a need for roofing granules that provide increased solar heat reflectance to reduce the solar absorption of the shingle, while providing a wide range of colors including deep-tone colors to maintain the aesthetic value of the system.